As the integration density of integrated circuits continues to increase, the physical dimensions of the feature sizes of the integrated circuits may continue to decrease. As is well known to those having skill in the art, integrated circuits generally include an integrated circuit substrate such as a monocrystalline silicon substrate in which active devices such as transistors are formed. A plurality of conductive layers are formed on the integrated circuit substrate, insulated from one another by insulating layers. Contacts are formed in contact holes in the insulating layers to selectively electrically connect the conductive layers to one another and to the underlying integrated circuit substrate.
As the feature size of integrated circuits continues to decrease, the size of the contact holes may also decrease. Moreover, the depth of the contact hole may increase or stay relatively constant so that the aspect ratio of the contact hole may increase. It may thus be increasingly difficult to form low resistivity contacts in these high aspect ratio contact holes.
More specifically, FIG. 1 illustrates a contact hole 3 with a sloped profile that is formed in an insulating layer 2 on an integrated circuit substrate 1. As the integration density continues to increase, it is generally desirable to provide a vertical profile contact hole 4 as shown in FIG. 2, to permit higher density contact holes. The use of a vertical profile, in addition to the high aspect ratio, may make it more difficult to form high performance contacts in the contact hole.
FIG. 3 is a cross-sectional view of a conventional integrated circuit contact formed using a conventional fabrication method. As shown in FIG. 3, a contact hole 14 is provided in an interlayer dielectric layer 12 that is formed on an integrated circuit substrate 10. A titanium (Ti) layer is formed in the contact hole 14 including on the contact hole bottom and the contact hole sidewall. A titanium nitride (TiN) layer is formed on the titanium layer to act as a glue layer or wetting layer. The combination of the titanium layer and titanium nitride layer form a barrier layer 16. The contact hole may then be filled by a metal such as tungsten (W).
Unfortunately, when filling the contact hole with tungsten, the tungsten hexafluoride (WF.sub.6) source gas may react with the underlying titanium layer or with a titanium silicide layer that is formed by reaction of the titanium layer with the integrated circuit substrate 10, to form an insulating layer such as a titanium fluoride layer. The titanium fluoride layer may increase the contact resistance and may cause stress due to volume expansion on the underlying integrated circuit substrate. The increased contact resistance and stress may increase the leakage current and/or cause other defects. Moreover, an edge portion 17 between the bottom and sidewall of the contact hole 14 may have a thin layer of the barrier metal 16 compared to other portions of the contact hole. Finally, when the titanium nitride layer is formed by sputtering, it may include columnar crystals, which may form columnar grain boundaries. These columnar grain boundaries may promote intergranular diffusion and thereby degrade the characteristics of the barrier metal layer.
In order to overcome these and/or other problems, Chemical Vapor Deposition (CVD) has been used to form the titanium nitride layer. Although CVD can provide a good step coverage, the CVD titanium nitride layer may be less dense, thereby degrading the characteristics of the barrier metal layer. Moreover, if titanium chloride is used as a source gas for forming the CVD titanium nitride layer, the chlorine in the source gas may attack the underlying integrated circuit substrate 10.
Alternatively, the titanium layer may be formed by Physical Vapor Deposition (PVD) methods such as sputtering. However, when using PVD, an undesirable titanium oxide layer may be formed. Accordingly, both the CVD process and the PVD process may introduce problems in forming high performance and reliable contacts for integrated circuit devices.